In recent years, studies and developments have been actively carried out to utilize liquid crystalline compounds as optical materials, and many of them have already been put in practical use. When a liquid crystalline compound is used as an optical material, it is necessary to retain the compound in its liquid crystal structure having been aligned and fixed, under practical service conditions. Various methods for fixing the liquid crystal structure have been proposed, such as those wherein polymerizable liquid crystalline compounds are used, wherein polymeric liquid crystalline compounds are used, and wherein polymeric liquid crystalline compounds having a crosslinkable reactive group are used.
In one of the methods wherein polymeric liquid crystalline compounds having a crosslinkable reactive group are used, there is used a polymerizable liquid crystalline compound comprising a mesogen composed of two or more benzene rings or similar rings, spacers each composed of a hydrocarbon chain, and radically polymerizable reactive groups such (meth)acrylate groups located at both or one of the terminal ends (see, for example, Patent Document 1 below). In this method, such a polymerizable liquid crystalline compound is applied after being heat-melted or in the form of a solution, on an alignment substrate and if necessary dried so as to be formed into a liquid crystal layer thereon, which layer is then heated to be aligned in a liquid crystal state and polymerized by photo irradiation to fix the layer in the aligned state. However, this method is required to inhibit the inhibition of polymerization caused by oxygen in the air, leading to the necessity of complicated operations such as photo irradiation under inert gas atmosphere and improvements in facilities and devices therefor. It is also necessary to pay meticulous care upon synthesis of the polymerizable liquid crystalline compound because the (meth)acrylate group is likely to be polymerized with light or heat.
As the method using a polymeric liquid crystalline compound, it is proposed to use a liquid crystalline polyester with excellent properties to retain the liquid crystal orientation (see, for example, Patent Document 2 below). However, as a result of the widespread of mobile devices, an optical film comprised of such a liquid crystalline polyester has been demanded to have properties to retain the liquid crystal orientation under more sever practical use conditions and more excellent mechanical strength as well.
As the method using a polymeric liquid crystalline substance having a polymerizable reactive group, there have been proposed a method wherein a polymerizable reactive group is introduced into the polymeric main chain and a method wherein monomer units having a polymerizable reactive group are introduced into the side chains. However, since in either of these methods the liquid crystallinity of the material used is decreased, there is a limit to the amount of the polymerizable group to be introduced to enhance the sufficient mechanical strength. Therefore, alternative methods have been demanded.                (1) Japanese Patent Laid-Open Publication No. 11-80081        (2) Japanese Patent Laid-Open Publication No. 11-158258        (3) Japanese Patent Laid-Open Publication No. 9-3454        